The present invention relates to oximeter sensors having a memory.
Pulse oximetry is typically used to measure various blood flow characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, and the rate of blood pulsations corresponding to a heart rate of a patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which passes light through a portion of the patient's tissue where blood perfuses the tissue, and photoelectrically senses the absorption of light in such tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured.
The light passed through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted or reflected light passed through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption. For measuring blood oxygen level, such sensors have been provided with light sources and photodetectors that are adapted to operate at two different wavelengths, in accordance with known techniques for measuring blood oxygen saturation.
Various methods have been proposed in the past for coding information in sensors, including pulse oximeter sensors, to convey useful information to a monitor. For example, an encoding mechanism is shown in Nellcor U.S. Pat. No. 4,700,708. This mechanism relates to an optical oximeter probe which uses a pair of light emitting diodes (LEDs) to direct light through blood-perfused tissue, with a detector picking up light which has not been absorbed by the tissue. The operation depends upon knowing the wavelength of the LEDs. Since the wavelength of LEDs can vary from device-to-device, a coding resistor is placed in the sensor with the value of the resistor corresponding to the actual wavelength of at least one of the LEDs. When the oximeter instrument is turned on, it first determines the value of the resistor and thus appropriate saturation calculation coefficients for the value of the wavelengths of the LEDs in the probe.
Other coding mechanisms have also been proposed in U.S. Pat. Nos. 5,259,381; 4,942,877; 4,446,715; 3,790,910; 4,303,984; 4,621,643; 5,246,003; 3,720,177; 4,684,245; 5,645,059; 5,058,588; 4,858,615; and 4,942,877, the disclosures of which are all hereby incorporated by reference. The '877 patent in particular discloses storing a variety of data in a pulse oximetry sensor memory, including coefficients for a saturation equation for oximetry.
Nellcor pulse oximeter sensors are encoded with a resistor (RCAL) value that corresponds to the wavelength(s) of the LED(s) within the emitter, such as described in U.S. Pat. No. 4,700,708. Nellcor pulse oximeter instruments read this resistor coding value and use it as a pointer to a look-up table that holds the proper set of coefficients for that sensor for calculating arterial oxygen saturation (SpO2). The function that converts the measured red and IR signal modulation ratio R (also known as the “ratio of ratios” or “rat-rat”) to a calculated saturation value is derived from the basic form of the Lambert-Beer Law:
                    R        =                                                            ln                ⁡                                  (                                                            I                      1                                        /                                          I                      2                                                        )                                            red                                                      ln                ⁡                                  (                                                            I                      1                                        /                                          I                      2                                                        )                                            ir                                =                                                                      S                  ·                                      β                    O2Hb                    red                                                  +                                                      (                                          1                      -                      S                                        )                                    ·                                      β                    Hb                    red                                                                                                S                  ·                                      β                    O2Hb                    ir                                                  +                                                      (                                          1                      -                      S                                        )                                    ·                                      β                    Hb                    ir                                                                        =                                                            S                  ·                                      c                    1                                                  +                                                      (                                          1                      -                      S                                        )                                    ·                                      c                    2                                                                                                S                  ·                                      c                    3                                                  +                                                      (                                          1                      -                      S                                        )                                    ·                                      c                    4                                                                                                          (        1        )            where I1 and I2 refer to detected light signals at two different points in the cardiac cycle, and the β's refer to the characteristic light absorption properties of oxygenated and deoxygenated hemoglobin. When solved for the saturation (S), the result takes on the form:
                              SpO          2                =                              S            ·            100                    =                                                                      c                  2                                -                                                      c                    4                                    ·                  R                                                                                                  (                                                                  c                        3                                            -                                              c                        4                                                              )                                    ·                  R                                +                                  (                                                            c                      2                                        -                                          c                      1                                                        )                                                      ·            100.                                              (        2        )            
Equation 2 can be further simplified to require only three constants (by, for example, dividing each constant by c2), but will be used as shown for the remainder of this description. Although theoretically based, the four constants c1-c4 are empirically determined. Theoretical values for the constants are insufficient primarily due to the complexities of light scattering and sensor optics. The values of the sets of constants (c1 through c4) vary with each resistor coding bin (each “bin” corresponding to a range of different characterized LED wavelengths). Multiple sets of coefficients (bins) are provided within a lookup table in Nellcor oximeters. When calculated SpO2 values according to Eq. 2 are less than 70%, a revised value of SpO2 using a linear function is used:SpO2=c5−c6.R,  (3)where both c5 and c6 vary with the resistor coding value. This linear function was found to better match SpO2 (arterial oxygen saturation as measured by a pulse oximeter) with SaO2 (the true value of arterial oxygen saturation, as measured directly on a blood sample) in observations made at low saturations.
A limitation of this method is that the proper calibration of the pulse oximetry sensor can be accomplished only if the relationship between the signal modulation ratio (R) to blood SaO2 conforms to one of the pre-encoded sets of calibration coefficients.
A further limitation of this method is that the relationship between R and SaO2 of the pulse oximetry sensor may not be linear in a low-saturation region, or that the breakpoint may not optimally be located at 70% SpO2.
A yet further limitation of this prior art method is that the functional relationship between the true arterial oxygen saturation and the measured signals may not fit a single function over the entire span of the measurement range.